So, what variables of the insulin pump need to be controlled? Which will be available to be "learned?"
Insulin pumps have been developed to compliment the teachings of endocrinologists and certified diabetes educators based on the concepts of intensive insulinotherapy (sometimes called flexible insulin therapy). This involves a flexible approach to regulating multiple doses of insulin per day, based on multiple blood glucose tests per day, and adjusting the dose based on foods about to be eaten, exercise or activities planned for the next few hours, and to correct for a deviation of the actual blood sugar right now.
This falls into 3 basic components of the insulin regimen:
- a correction dose based on the difference between actual BG level and a target BG level, divided by a correction factor of insulin sensitivity (in BG counts per unit of insulin, or more correctly mg/dl/unit)
- a meal bolus or a single large dose of insulin to cover a meal about to be eaten, based on a count of carbohydrate grams of the food multiplied by the insulin-to-carb-ratio (I/c) in units per gram.
- a basal insulin, or a slow release (background) insulin that a person needs all the time. New insulin analogs such as Lantus and Levimur last for 12 to 24 hours and give a low slow dose of background insulin.
A formula often taught to diabetic children to calculate their insulin requirement before a meal is:
premeal insulin injection (in units) = Correction dose + meal bolus
where the correction dose = (((Actual BG level) - (target BG level)) / (Insulin sensitivity factor in BG counts per unit)
and the meal bolus = (grams of carbs in food about to be eaten) / (Insulin to carb ratio for that meal)
Here are the key variables in the algorithm, and their definitions:
- Target Value: the desired intermediate value (often 110 mg/dl) used to calculate a precise correction bolus (as opposed to the upper and lower values of a target range)
- Insulin Sensitivity Factor: the ratio of the expected impact of insulin on the blood sugar given in mg/dl per unit of insulin. Example, one persons sensitivity might be a drop in 60 mg/dl of blood sugar per unit of insulin given.
- Insulin-to-Carb Ratio: the ratio of grams of glucose covered by one unit of insulin, usually given as grams per unit. The ratio is specific to one meal for that one diabetic person (breakfast = 15g/u, lunch = 19g/u, etc)
- Basal Rate: the rate of continuous insulin delivery for basal needs equivalent to the total daily basal insulin such as from Lantus or Levimur
Basal insulin is dosed in units needed for a 24 hour period. Often this accounts for half of the total daily insulin requirement, depending on the person and their daily activities. It can be one injection per day, but is sometimes broken down into two injections per day. Lantus for example might be dosed as one injection of 24 units, which is relative to 2 injections of Levimur at 12 units each, and related to a continuous dose of 1 unit per hour. Each has the same total dose per 24 hour period.
Not surprisingly, the insulin pump and its model for continuous subcutaneous insulin infusion matches closely the same principals used in intensive insulinotherapy. It's primary difference is in the replacement of long lasting basal insulin with a continuous push of short-acting insulin. In fact, the insulin pump delivers one choice of insulin (the user makes the choice) for both basal and bolus needs. The insulin profiles (action over time) of the long acting insulin are not ideal. Ideal is defined here as a "flat" or peakless profile of insulin release over time. Ideal may not necessarily match the needs of a single insulin user, but it gives a stable baseline for which individual insuln users can customize for their particular needs. The problem, well known of course, is that n one of the long lasting insulins are really peakless. While Lantus and Levimur profiles are a leap ahead over the older NPH strong peak profile, and while they are very usable right now, the long lasting insulins are still not ideal. Enter the insulin pump. Compared to the long acting insulins, the continuous infusion of fast-acting insulin for basal needs offers these benefits:
- Accurate low rate delivery making CSII possible,
- Elimination of the peaks and valleys of those insulin profiles,
- The dynamic ability to dial back the basal insulin to react to an impending low blood sugar or to avoid hypoglycemia due to exercise or spontaneous activities.
- Control and memory of the basal profile, to actually "fit" the basal profile to the specific needs of individuals.
The same features that make for a good fit of basal insulin also benefit the pump user for all insulin needs:
- accurate small boluses, as needed for more precise carb counting
- precise correction doses, allowing partial units (0.05 to 0.1 units) and lower BG targets instead of the upper level of a target range
- achieving tighter blood glucose control, which ultimately realizes a lower HbA1c
Therefore, the algorithm for interfacing with the insulin pump is the same as with intensive insulinotherapy, but utilizes a basal rate of fact-acting insulin. The key parameters, as shown in the "data store" boxes in the diagram above, are all needed to control the insulin pump and can be "learned" by the insulin pump with the use of adaptive variables.